Automatic calibration mode for carbon dioxide sensor

ABSTRACT

An automatic calibration mode for a carbon dioxide sensor is based on using a zero gas. The sensor is designed to recognize a distinctive rate of change pattern of carbon dioxide concentrations that would be indicative of a zero calibration routine. This approach can be used for other infrared sensors.

PRIORITY

This application is related to application Ser. No. 09/004,142, entitled“Method for Detecting Venting of a Combustion Appliance Within AnImproper Space”, filed Jan. 7, 1998, the disclosure which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of sensortechnology. More specifically, the present invention relates to thedevelopment of various infrared (IR) gas sensor technology applicationsin connection with carbon dioxide sensing, particularly as tomeasuring/controlling exhaust gas recirculation (EGR) to diesel engines.

BACKGROUND OF THE INVENTION

To acquire desired information of any kind, measurements of physicalparameters must be made. Devices that permit these measurements arebroadly categorized as sensors. The term “sensors” encompasses a broadrange of technologies and devices that respond to a physical stimulus(i.e. light, heat, sound, pressure, magnetism or a particular motion)and transmit a resulting impulse, generally for measurement or operatinga control.

Sensors are widely used in many different applications. Some can be assimple as the direct measurement of a thermocouple, or as complex asall-weather imaging systems. Whatever the complexity of the sensor, aninteraction between the sensor and its physical environment producessome kind of signal that ultimately leads to the desired information.

In many instances, sensor technology has become a basic enablingtechnology. The rapid increase in the interest in sensors has beendriven by numerous applications, such as analysis of selected compoundsin blood, in which sensors can provide a large public benefit.

In addition, sensors are of great importance in safety-related areas,with applications ranging from assessing the integrity of aircraft tofire safety monitoring. Common research and technology issues in thesediverse applications include the interpretation of spectral signaturesin terms of quantities of interest, such as concentrations, temperaturesor thermal properties.

For example, market demand in gas measurement platforms for determiningconcentration levels of carbon dioxide, is driving increased activity inCO₂ technology because in part of its utility in understanding andmonitoring ventilation and Indoor Air Quality (IAQ). Building codes andstandards governing ventilation in buildings, such as ASHRAE (AmericanSociety of Heating Air Conditioning and Refrigeration Engineers)Standard 62-99, have established minimum volumetric outside airrequirements on a per-person basis.

Because individuals generally exhale a predictable amount of carbondioxide, one application is to use this parameter to sense occupancy. Anincreasing or decreasing level of carbon dioxide can indicate ingress oregress of an indoor zone.

In addition, because outside levels are very low and constant, an indoormeasurement can also provide a dynamic measure of the number ofoccupants of the space and the amount of low concentration outside airbeing introduced to dilute contaminant concentrations. As a result, acarbon dioxide measurement in a space can be used to measure or controlper-person ventilation rates within a space.

Thus, while carbon dioxide is not a direct measure of indoor airquality, it has the potential to be an excellent measure of effectiveventilation (mechanical ventilation plus infiltration). Generally, thehigher the carbon dioxide concentration, the lower the ventilation. Inother words, when indoor carbon dioxide levels are very high (i.e. above1800 ppm) and ventilation is low (below 7 cfm/person), these conditionscan allow contaminants to build up, causing irritation and discomfort.

Determining concentration levels of carbon dioxide is a powerfulfacility that can be useful in homes, office buildings, schools, andother commercial environments. However, implementable applications arelimited by manufacturing and other costs, as well as health, safety,quality and other issues.

For example, in health and safety applications, oxygen sensors have beenused to measure depletion of oxygen. However, oxygen sensors are notonly expensive, but generally require periodic replacement orrecalibration. Thus, it would be desirable to have an inexpensivealternative sensing method of measuring oxygen depletion.

In the automotive industry, there is also an increasing need for carbondioxide sensor technology to improve the quality, safety and comfort ofautomobiles. For instance, it is known that the carbon dioxideconcentration in the combustion air to an engine can be used todetermine the amount of exhaust gases being re-introduced to theengine's combustion air. This is because the carbon dioxideconcentration of the engine exhaust is significantly higher than ambientair (i.e. 9 percent versus 350-550 ppm).

However, conventional sensing approaches for gases in engines utilizein-situ sensors that are directly exposed to the stream of gas beingmeasured. Exposing these types of sensors to the harsh engineenvironment, particularly high temperatures, impairs sensing quality andresults. Thus, it would be desirable to have an alternative sensingapproach, to determine carbon dioxide concentrations, that could endurethe harsh environment in the engine and still produce accuratemeasurements.

An equally important driver in the automotive industry is theincorporation of sensors into automotive products that aid in extendinghuman life and improving safety. In one instance, there is a need tosense the presence of an individual within a vehicle's trunk in order toprevent unwanted or accidental confinement that could lead to death.

In the area of sensor recalibration, there is an increasing need forsensors with an automatic calibration mode feature that has a fastrecalibration time, and provides stable, false-free readings.

Accordingly, there is a need for an inexpensive sensor technologycontrol approach that can be used as an indicia of concentration levelcharacteristics of carbon dioxide. In addition, there is a need for acontrol approach that is suitable (i.e. standardizable) across a numberof different applications.

SUMMARY OF THE INVENTION

The foregoing and other needs have been satisfied to a great extent bythe present invention, which includes a very reliable method ofdetermining concentration levels of gases, such as carbon dioxide.

More specifically, the present invention is achieved by use of a gasmeasurement criterion based on measuring the rate of change of carbondioxide concentration and variations thereof, using optical methods.Optical methods are the most accurate and reliable method for measuringcarbon dioxide because of its inert nature; carbon dioxide reacts poorlywith other gases, and is difficult to measure reliably with sensors thatdepend on physical or chemical reactions.

In one aspect of the present invention, a method of measuring oxygendepletion is employed using the rate of change of carbon dioxide as asurrogate indicator of the amount of oxygen being depleted or displacedin the air. Depletion of oxygen can be measured in one of two instances.

In a first instance, if oxygen is being displaced by carbon dioxide, thenatural consequence is that carbon dioxide will have to rise to veryhigh levels to displace a significant amount of oxygen (e.g. greaterthan 30,000 ppm or 3 percent of CO₂).

Conversely, and in a second instance, if oxygen is being displaced byanother gas, then it follows that concentrations of carbon dioxide willultimately begin to drop below normal atmospheric levels of 350-450 ppm.If the rate of fall of CO₂ levels drops to below 300 ppm within 24 hoursor less, for example, such a drop is reasonably indicative of oxygendepletion, and a warning or control is triggered.

More accurate control levels can be established given known space volumeinformation. Also, the rate of change of carbon dioxide can be used ifthe rate of change exceeds normal rates that could be expected to begenerated by human occupants.

In another aspect of the invention, a carbon dioxide sensor is remotelyconfigured in an automotive or diesel engine in order to measure andcontrol exhaust gas recirculation (EGR) to diesel engines. EGRtechniques are used to reduce the emission of certain pollutants, suchas nitrogen oxide (NOX), to meet EPA or other environmentalrequirements.

In order to maintain optimum operating conditions of the engine, and atthe same time reduce emissions such as NOX, the ratio of exhaust gassesrecirculated into the engine air intake to fresh outside air introducedto the engine, must be relatively constant. Maintaining this ratio canbe difficult because of the varying operating speed and correspondingcombustion air requirements of the engine.

This ratio may vary with engine design, but typically is approximately20 to 25 percent EGR to fresh air. Since outside air has very lowconcentrations and engine exhaust has very high concentrations (i.e.9-12 percent by volume), the carbon dioxide concentration in the mixedair compartment can provide an indication of the outside air to EGR airmixing, and can be used to maintain the appropriate EGR ratio. Thisapproach employs a sampled method to determine carbon dioxideconcentrations.

In this approach, a conductive sample tube is installed into theengine's pre-combustion, air-mixing chamber and channels air to a remoteCO₂ sensor. This chamber is an area where exhaust gases are combinedwith ambient air before being introduced to the engine for combustion.This chamber may be inside the engine or as part of an assembly attachedto the engine.

The conductive sample tube is of sufficient length to allow foradditional cooling of the sample so that sampled gas temperatures areless than 50 degrees Centigrade, but not too long as to cause asignificant delay in response time. Sampled air from the pre-combustionair-mixing chamber is pushed through by the pressure differentialbetween the chamber (e.g. 1 atmosphere or more) and ambient pressuresaround the engine.

Optionally and alternatively, this sampling approach can facilitatecontrol strategies for removing particulates, if necessary, that mayrequire filtration. In addition, reducing the temperature of the sampleallows for less complex and potentially more inexpensive CO₂ sensortechnology to be used, by eliminating the need for components andcalibration that operate in high temperature environments.

Another feature and advantage of this aspect of the invention ispresented if a conditioned sample is used, because it enables easiermeasurement of other gases, such as NOX, that are more easily measuredusing optical methods at temperatures below 50° C.

In a third aspect of the present invention, the rate of change of carbondioxide concentration levels is used to indicate the presence of anindividual in an automobile trunk. There are several other sources ofcarbon dioxide that must be factored into consideration, since thesesources may affect an accurate CO₂ sensor reading. The two primarysources are: (1) leakage of carbon dioxide exhaust into the trunkcompartment, and (2) deliberate injection of carbon dioxide into thetrunk compartment to intentionally activate the sensor; perhaps toautomatically open the trunk.

To guard against the occurrence of the two above-mentioned conditions,the CO₂ sensor of the present invention and its warning/control logic ispreferably configured to identify a rate of change of carbon dioxidethat may be likely when one or more humans become trapped in a trunk.

Human carbon dioxide production is a function of activity and body size.Thus, an action/control point within the control logic is calculablebased on rate of change data involving, for example, possible ranges ofcarbon dioxide production rates for assumed range or activity level(s).An action/control point within a control logic can also be calculatedbased on rate of change data, which calculations include, for example,the volume of the trunk; assumptions for air leakage rates for thetrunk; and occupant ages.

These calculations preferably provide a carbon dioxide rate of changerange in which it could be determined with reasonable accuracy that ahuman occupant is in the trunk. If the rate of change measured withinthe trunk falls within the calculated range, desired control or alarmstrategies can be activated. Control or alarm strategies may include anindicator light, buzzer, opening of the trunk, flashing lights, soundinghorn, and the like. If suspicious CO₂ levels were detected outside thedesired or predetermined sensing range, an alternative indicator can beactivated.

In a fourth aspect of the present invention, an automatic calibrationmode for carbon dioxide sensors is provided. Calibration is based onusing a zero gas for calibration. This approach can be used for otherinfrared sensors, besides carbon dioxide sensors, as well.

Given that ambient levels of carbon dioxide are generally 350 ppm orhigher, and that carbon dioxide levels tend to change gradually within aspace, the CO₂ sensor of this aspect of the invention is designed torecognize a distinctive rate of change pattern of carbon dioxideconcentrations that would be indicative of a zero calibration routine.Once this distinctive pattern is identified, the sensor is triggered togo into a calibration mode and reset its calibration based on the CO₂concentrations being measured during the calibration mode.

In a preferred embodiment, the distinctive pattern may comprise adramatic drop of 200 to 300 ppm or more over approximately 15 to 30seconds in the sensor reading. Another distinctive pattern may comprisea stable reading for the subsequent 30 seconds, indicating a constantflow of the calibration gas to the sensor. This pattern may optionallyactivate the calibration mode of the sensor. Provided that carbondioxide concentration levels remained stable through the calibrationperiod (e.g. 1 to 5 minutes), the sensor may recalibrate itself to zerobased on the same gas it is measuring.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein as well as the abstract included below are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting carbon dioxide concentration values againsttime, in accordance with the rate of change methodology of the presentinvention.

FIG. 2 is a flow chart showing the decision logic for determining thepresence of an individual in a space, according to one aspect of thepresent invention.

FIG. 3 is a flow chart illustrating the decision logic for determining acondition of oxygen depletion, using a variation of the rate of changemethodology of the simulation model of the present invention.

FIG. 4 is a flow chart illustrating the decision logic of an automaticcalibration mode for a carbon dioxide sensor, in accordance with anotheraspect of the present invention.

FIG. 5 is a perspective view of an engine's chamber showing use of acarbon dioxide sensor to measure and control exhaust gas recirculationto a diesel engine, according to another aspect of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Carbon dioxide is present in outside air, but generally at very lowconcentrations; namely, from approximately 350 to 450 parts per million(ppm). People are the major contributors of carbon dioxide insidebuildings, and can contribute enough carbon dioxide to allow levels torise as high as 3,000 ppm if a space is poorly ventilated. Concentrationlevels of carbon dioxide can be higher, but such elevated concentrationsare unusual under normal circumstances.

Certain characteristics of carbon dioxide concentration in a space canbe indicative of health- and/or safety-risk conditions. Thesecharacteristics include an achievement of an absolute threshold level,an observed increase of CO₂ levels occurring in conjunction with anotheractivity, or a rate of change of CO₂ levels over a period of time.

The gas measurement technique of the present invention, employing therate of change concept to determine concentration levels of gases in aspace, finds utility in many different real-life applications. Thisrationale holds true whether the rate of change methodology is used as asole criterion or in combination with other measurement techniques.

One aspect of the invention that utilize rate of change as a solecriterion is exemplified in the home. For most common occupancies inresidential structures, for example, modeling studies show that humanscontribute carbon dioxide levels that will result in CO₂ levels of lessthan a 5 ppm per minute rate of change.

In contrast, similar modeling studies show that a furnace flue(generally containing between 90,000 and 120,000 ppm CO₂), if venteddirectly into a structure, can contribute to a rate of rise of carbondioxide levels from 20 to 100 ppm per minute, or more. Thus, dependingon the location measured and the volume of enclosed space, rates of risein carbon dioxide concentration is an accurate indicator of the presenceof combustion fumes and the like.

It is important to note that a controlled combustion process, such as afurnace, would provide a relatively constant rate of change duringoperation. This is in contrast to an uncontrolled fire, which wouldprovide a steadily increasing rate of change of carbon dioxide levels asthe uncontrolled fire increased in size.

Depending on the volume of the space, rates of rise of carbon dioxideconcentration is also an accurate indicator of combustion leaks. Forexample, a rate of rise of carbon dioxide of over 30 to 50 ppm perminute is indicative of a furnace or fireplace combustion product(s)leaking into the living space of a house. This type of measurement canbe made in the general living space of the house, in the furnace room,utility room or in a garage attached to the house. Alternatively andoptionally, a carbon dioxide threshold rate of rise level ofapproximately 25 ppm per minute can be used as the criterion indicativeof improper venting.

In accordance with a first aspect of the present invention, the rate ofchange of carbon dioxide concentration is used as an accurate criterionto indicate the presence of one or more individuals in a space. Thiscriterion could also be useful in detecting the approximate number ofpeople contained within an enclosed container in applications wheresmuggling of people may be involved. In addition, if the space is avehicle trunk compartment, the prevention of deaths by humans beingtrapped in the trunk, mistakenly or otherwise, is another advantageousapplication of the present invention.

More specifically, to determine rate of change of carbon dioxideconcentration characteristics, the Applicant derived a simulation modelthat considers a number of factors in calculating the resulting CO₂rates of change that occurred over a period of time. The factors includevolume of space, air leakage of the space and, if any, other sources ofcarbon dioxide that may enter the space.

Another set of factors the simulation model takes into account is humancarbon dioxide production rates. The rate of carbon dioxide productionby humans is related to both the size of the individual, including age,and activity level. For instance, an adult (i.e. ages 14-65) in aresting mode generally produces a constant rate of CO₂ at approximately0.25 liters per minute. Production rates in this range is likely tooccur when people are hidden in a confined space. For an individuallocked in a car trunk, activity levels would be higher and thereforeproduction rates may range from 0.5 to 1.5 liters per minute.

The rate of change methodology calculation employed in the simulationmodel of the present invention looks at the increase of carbon dioxideover time taking into consideration, for example: (1) the number ofpeople in the space, and their expected age and activity level; (2) thevolume of the space; and (3) any natural or mechanical methods ofventilation that may dilute space concentrations with outside air. Theserelationships obey the following equation:

 C _(t)=[(C _(t−1) +N/V−C _(OA))(1−A/V)]+C _(OA)

where:

C_(t)=CO₂ concentration at time t (ppm);

C_(t−1)=CO₂ concentration in previous time period (ppm);

C_(OA)=CO₂ concentration in outside air (ppm);

N=CO₂ generation rate by human occupants (cfm);

V=space volume (cu. ft.);

A=the volume of dilution air introduced into the space.

Using the equation above, a rate of change profile is developed for arange of probable/desired situations involving the presence of one ormore individuals in a space. When the rate of change data is determinedfor one or all of the above-mentioned situations, a range of CO₂concentration values is developed to indicate what a likely CO₂ rate ofchange level would be for a given trunk or enclosure space.

This range is achieved by calculating low bound and high bound carbondioxide concentration values for varying changes in, for example, spacevolume, activity level, outside carbon dioxide level and/or ventilation.When carbon dioxide concentration values are plotted against time, thegraph of FIG. 1 is produced. The area between high and low bound valuesis best described as an alarm zone in which one or more alarm strategiesbecome activated when concentration levels of carbon dioxide fallstherein.

Referring now to FIG. 2, there is shown a flow chart of the decisionlogic describing a process for determining the presence of an individualin a space, using the rate of change methodology of the simulation modelof the present invention.

As previously discussed, the process begins with a determination ofgenerally accepted leakage conditions of the space, (Step 20 or S20). Inone embodiment, the space comprises a vehicle trunk compartment. Inanother embodiment, the space may comprise a shipping container or thelike. Where the space is an automotive trunk compartment, for example,leakage of carbon dioxide exhaust into a trunk compartment isdetermined.

Once “normal” leakages have been determined, the next step is todetermine or calculate all other appropriate CO₂ leakages into thespace, (S22). For example, the deliberate injection of carbon dioxideinto the trunk compartment may be included in the possibilities of otherappropriate leakage conditions, since it may be performed tointentionally activate the sensor to open the trunk. Other suchpossibilities should be determined.

To guard against the occurrence of these and other possibilities or toprevent unwanted false alarms created by these possibilities, awarning/control logic is preferably configured to incorporate theselikely and/or unwelcomed conditions. In essence, the warning/controllogic is configured to identify a rate of change of CO₂ levelsrepresentative of one or more trigger value(s), (S24) such as when oneor more humans become trapped in the trunk or enclosure space.

In a preferred embodiment, this identification is accomplished in partby employing one or more action/control point(s), which is/arecalculated based on rate of change data. The rate of change dataincludes calculations on, for example, volume of trunk information;assumptions for air leakage rates for the trunk; carbon dioxideproduction rates (e.g. exhalation per minute data); ranges of carbondioxide production possible for an assumed range or activity levels(e.g. low versus high activity level exhalation/minute data); andoccupant age (e.g. adult versus a child).

These calculations preferably provide a carbon dioxide rate of changecriterion useful for determining with reasonable accuracy the rate ofchange of carbon dioxide within the space (S26) and ultimately whether ahuman occupant is in the vehicle trunk compartment or enclosure space.

If the rate of change of carbon dioxide concentration levels within thespace falls outside a calculated range, (i.e. the trigger), (S28) theprocess repeats itself. However, if CO₂ concentration rate of changelevels falls within the trigger, one or more control or alarm strategiesare activated (S30).

In a preferred embodiment, an alarm is sounded if the rate of change ofcarbon dioxide of at least approximately 50 ppm per minute is detectedin the trunk compartment or enclosure space. The alarm may be in theform of an indicator light.

The desired trigger rate-of-change value(s) for generating an alarmsignal during a desired period is changeable and very dependent on spacevolume. For instance, the control or alarm strategy may be used so asnot to trigger an alarm upon a single occurrence of detection of a rateof change of carbon dioxide, but only once a rate of change is detectedover a consecutive number of times. By not triggering an alarm signaluntil a rate of change is detected over two or more periods, forexample, or even a larger number of periods, such as five or more, thepossibility of false alarms can be further minimized.

Alternatively and optionally, different alarm signals may be generatedfor different trigger value(s). For instance, an initial alarm signalmay be generated if a rate of change greater than approximately 5 ppmper minute is detected, a second alarm may be generated if a rate ofchange greater than approximately 10 ppm per minute is detected, and/ora third alarm generated if a rate of change greater than approximately25 ppm per minute is detected. Each alarm may take the form of a buzzer,opening of the trunk compartment, flashing lights, sounding horn, andthe like.

Audible alarm signals may also be valuable in interpreting why the alarmsignal was generated. For example, if suspicious rate of change value(s)were detected outside the sensing range of the trunk compartment, a verylow audible alarm signal may be activated to indicate a low existencelevel.

In yet another aspect of the present invention, the above rate of changeconcept is employed to indicate a condition of oxygen depletion.Referring now to FIG. 3, there is shown a flow chart showing the processfor determining a condition of oxygen depletion, using a variation ofthe rate of change methodology of the present invention. The idea hereis to use the rate of change of CO₂ concentrations rising above orfalling below a level that indicates the displacement of oxygen.

The process begins with a determination of all pertinent carbon dioxideleakage conditions into the space (S40). The space may comprise anyvariety of enclosure spaces, such as a room in a house, office orcommercial building, an auditorium, or the like. Pertinent leakageconditions are user- and situation-definable.

The next step is to determine the carbon dioxide rate of change levelthat indicates displacement of oxygen (S42). For instance, carbondioxide would have to rise to very high levels (e.g. >30,000 ppm) inorder to observably displace a significant amount of oxygen, if oxygenis being displaced by carbon dioxide.

Alternatively and conversely, concentrations of carbon dioxide droppingat a rate below normal atmospheric levels of 350 to 450 ppm, ispreferably indicative that oxygen is being displaced by another gas. Anyconsistent and sustained drop below these levels would indicatedisplacement of carbon dioxide. In this fashion, carbon dioxide operatesas a surrogate indicator of the amount of oxygen being depleted ordisplaced in a space.

A subsequent step is to determine the rate of change of carbon dioxideconcentration of the space (S44). Note that the above steps need not beperformed in the exact sequential order as discussed, just as long aseach determination is made prior to Step 46 (S46), which is an inquirywhether the CO₂ rate of change of the space is above or below the CO₂rate of change level.

A warning/control logic is also configurable to identify either a carbondioxide rate of change range or threshold level that is indicative ofoxygen displacement. The rate of change data may include the following:volume of space information; assumptions for air leakage rates; carbondioxide production rates; ranges of carbon dioxide production forassumed ranges or activity levels; and occupant age(s).

Moreover, once an undesired rise or drop has been determined, a warningor control alarm is optionally triggered (S48), as previously discussed.The warning or alarm may take the form of a visible or audible signal.

More accurate control levels can be established given known space volumeinformation. Space volume is a critical factor in calculating rate ofchange value(s). Also, the rate of change of carbon dioxideconcentration can be used if the rate of change exceeds normal ratesthat could be expected to be generated by human occupants.

In yet another aspect of the present invention, an automatic calibrationmode for a carbon dioxide sensor is provided. Automatic calibration incarbon dioxide or other infrared sensor is based on using a zero gas forcalibration. Referring to FIG. 4, the idea is to inject a zero gas intothe sensor (S52), and look for a dramatic drop in carbon dioxideconcentrations (S54) below a given or desired baseline for sensoroperation (S50). (Atmospheric concentrations of carbon dioxide below 350ppm ate generally unlikely.) The zero gas is preferably not a gasgenerally tested by the sensor, such as nitrogen.

The dramatic drop may be determined as desired. In a preferredembodiment, a drop may be determined to be 200 ppm or more over afive-minute period. In other words, if the drop is more than 200 ppm(S56) over the designated or desired time period, the sensor isconfigured to automatically recalibrate itself (S58) with amicroprocessor-driven switch that switches on. In essence, the dramaticdrop in sensor reading over the course of five minutes, would beindicative of the zero gas and the sensor would recalibrate that readingto be the zero point.

In another embodiment, the dramatic drop may be determined to be a rangeat approximately 200 to 300 ppm or more over approximately 15 to 30seconds in the sensor reading. Alternatively and optionally, thetriggering condition may comprise a stable reading for the subsequent 30seconds, indicating a constant flow of the calibration gas to thesensor. This pattern may optionally activate the calibration mode of thesensor. Provided that carbon dioxide concentration levels remainedstable through the calibration period (e.g. 1 to 5 minutes), the sensormay recalibrate itself to zero based on the same gas it is measuring.

A diagnostic routine can also optionally be performed to determine thestability of the reading over a period of time, such as one minute.Preferably, the variation in reading of the measurement should notexceed the background noise of the sensor.

In another aspect of the present invention, the rate of change of carbondioxide concentration is also measurable in the recirculated air of anengine's precombustion, air-mixing chamber 10. As shown in FIG. 5,chamber 10 is simplified to include those elements pertaining to an easyfacilitation of the present invention.

A conductive sample tube 12 is installed into the engine'spre-combustion chamber 10 to channel air from the chamber 10. In chamber10, exhaust gases are combined with ambient air before introduction tothe engine (not shown) for combustion. Also, chamber 10 may be arrangedinside the engine or as part of an assembly attached to the engine.

The length of the sample tube 12 is important. In a preferredembodiment, the sample tube 12 is of sufficient length to allow foradditional cooling of the sample air, so that sampled gas temperaturesare less than 50 degrees Centigrade, but not too long as to cause asubstantially significant delay in response time. Sampled air from thechamber 10 is pushed through the tube 12 by the pressure differentialbetween the chamber 10 (i.e. 1 atmosphere or more) and ambient airpressure around the engine.

A carbon dioxide sensor 14 is configured onto the conductive sample tube12 a distance remote from the chamber 10. By this arrangement, thecarbon dioxide concentration in the pre-combustion air to an engine isaccurately measured despite the harsh engine environment.

Alternatively and optionally, this arrangement can be useful forremoving air particulates, if necessary, that may require filtration. Inaddition, in cases where the temperature of the sample may be reduced,less complex and therefore more inexpensive, non-temperature-sensitivecarbon dioxide sensor technology can be used, because the need forcomponents and calibration that operate in high-temperature environmentsis eliminated.

Moreover, in other cases where a conditioned sample is used, thisarrangement of the present invention may facilitate easier measurementof other gases, such as nitrogen oxide (NOX), that are more easilymeasured using optical techniques at temperatures below 50 degreesCentigrade. That is, calculation of the NOX level of the exhaust gas canbe determined based on the NOX measured in the chamber 10 and thepercentage of exhaust gas recirculation (EGR) being introduced to thechamber 10 calculated from carbon dioxide levels. In other words, bykeeping the percentage of EGR at a certain amount, the level of NOX canbe maintained at no more than a certain level.

It is important to recognize that in measuring the rate of change ofcarbon dioxide concentration, it is not necessary to measure absoluteconcentrations. The critical point is the relative change inconcentrations. In order to minimize costs, it is contemplated that aNon-Dispersive Infrared (NDIR) sensor be used as the carbon dioxidesensor when practicing an embodiment of the present invention. NDIRdetectors responds well to CO₂, have better than average sensitivities,and lasts longer than electrochemical detectors.

Accordingly, it would also be possible to use a carbon dioxide detectorthat relies upon a ratioing technique to determine when a rate of changein carbon dioxide exceeds what would be equivalent to a desired level. Adescription of a ratioing technique used in an NDIR sensor is set forthin U.S. Pat. No. 5,026,992, the disclosure of which is incorporatedherein by reference.

The above description and drawings are only illustrative of preferredembodiments which achieve the objects, features, and advantages of thepresent invention, and it is not intended that the present invention belimited thereto. It would be readily apparent to those skilled in theart that further changes and modifications in the actual conceptsdescribed herein can be readily made without departing from the spiritand scope of the invention as defined in the following claims.

For example, it should be possible to design a single apparatus thatcould be used to practice the invention that includes various logicoptions and/or is programmable to reset or revise such logic options.Also, the alarm signal can take on any of a variety of forms, such as anaudible or visible warning.

Or the alarm signal could be an electrical signal sent to a device thatacts upon receipt of the alarm signal. Thus, for example, generation ofthe alarm signal could trigger another event, such as turning off theengine of a vehicle or altering its operation. Alternatively, anotherdevice or process could be triggered into operation to remove airparticulates.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirits and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. A method for automatic calibration of a carbondioxide sensor, comprising the steps of: determining a baseline carbondioxide concentration value for sensor operation; injecting a zero gasinto said sensor; determining whether at least one drop in carbondioxide concentration value occurs below the baseline value over apredetermined time period; and activating recalibration of said sensorwhen said drop in carbon dioxide concentration occurs.
 2. The methodaccording to claim 1, further comprising: recognizing a predeterminedrate of change pattern of carbon dioxide concentrations.
 3. The methodaccording to claim 2, further comprising: resetting the sensor'scalibration, once said predetermined rate of change pattern isidentified, based on the carbon dioxide concentration measured.
 4. Themethod according to claim 1, wherein the step of determining thebaseline carbon dioxide concentration value for sensor operation isuser-definable.
 5. The method according to claim 1, wherein the step ofdetermining the drop in carbon dioxide concentration over saidpredetermined time period is user-definable.
 6. The method according toclaim 1, wherein said drop is 200 parts per million over a five-minuteperiod.
 7. The method according to claim 1, wherein said drop includes arange from 200 to 300 ppm over 15 to 30 seconds.
 8. The method accordingto claim 1, wherein said drop in sensor reading over a five-minute timeperiod is indicative of a zero gas, causing the sensor to recalibratesaid reading to be a zero point.
 9. The method according to claim 1,wherein the step of activating recalibration includes resetting thesensor's calibration with a microprocessor-driven switch.
 10. The methodaccording to claim 1, further comprising: performing a diagnosticroutine to determine stability of one or more sensor readings overanother predetermined period of time.
 11. The method according to claim1, further comprising: activating an alarm strategy when the step ofactivating recalibration occurs.
 12. The method according to claim 11,wherein said step of activating an alarm strategy includes activating atleast one of a visible and audible alarm.
 13. The method according toclaim 11, wherein said step of activating an alarm strategy includesactivating at least one of a visible and audible alarm upon a singleoccurrence of resetting of the sensor's calibration.
 14. The methodaccording to claim 11, wherein said step of activating an alarm strategyincludes activating at least one of a visible and audible alarm upon astable reading for a 30-second period, said stable reading indicating aconstant flow of a calibration gas to said sensor.
 15. The methodaccording to claim 1, wherein a variation in sensor reading of saidcarbon dioxide concentration being measured does not exceed a backgroundnoise of said sensor.
 16. The method according to claim 1, wherein saidzero gas is nitrogen.
 17. A system for automatically calibrating acarbon dioxide sensor, said system comprising: means for determining abaseline carbon dioxide concentration value for sensor operation; meansfor injecting a zero gas into the sensor; means for determining whetherat least one drop in carbon dioxide concentration value occurs below thebaseline value over a predetermined time period; and means foractivating recalibration of the sensor when said drop in carbon dioxideconcentration occurs.
 18. The system according to claim 17, wherein themeans for determining the baseline carbon dioxide concentration valuefor sensor operation allows a user to define said baseline carbondioxide concentration value.
 19. The system according to claim 17,wherein the means for determining the drop in carbon dioxideconcentration over said predetermined time period allows a user todefine said drop in carbon dioxide concentration over said predeterminedtime period.
 20. The system according to claim 17, further comprising:means for activating an alarm strategy when said means for activatingrecalibration are used.
 21. The method according to claim 1, furthercomprising: means for recognizing a predetermined rate of change patternof carbon dioxide concentrations.
 22. An apparatus for automaticcalibration of a carbon dioxide sensor, comprising: a detector fordetermining a baseline carbon dioxide concentration value for sensoroperation; an injector for injecting a zero gas into the sensor, saidinjector in communication with the sensor; a microprocessor, whereinsaid microprocessor determines whether at least one drop in carbondioxide concentration value occurs below the baseline value over apredetermined time period, said microprocessor in communication withsaid detector; and a switch wherein, said switch switches on to activaterecalibration of the sensor when said drop in carbon dioxideconcentration occurs, said switch being in communication with saidmicroprocessor and the sensor.
 23. The apparatus according to claim 22,wherein said detector for determining a baseline carbon dioxideconcentration value for sensor operation, accepts user-defined input.24. The apparatus according to claim 22, wherein the said microprocessoraccepts user-defined input.